A lack of consensus on the contribu

A lack of consensus on the contribution of cold-season soilrespiration to the annual ABZ carbon cycle and the potential carbon feedbacks of ABZ ecosystems to global climatecan be largely attributed to a relatively poor understandingof changes in liquid water content and soil thermal regimethat occur during the seasonal soil freeze–thaw (F–T) transition (Oechel et al., 1997; Zona et al., 2016). Models typically assume that the thaw or growing season is the mostactive period of carbon exchange in ABZ ecosystems, whilesoil respiration largely shuts down when surface soils freeze(Commane et al., 2017). However, unfrozen conditions indeeper soil layers can persist for a substantially longer period than surface soils and maintain a significant amount ofliquid water, sustaining soil respiration for several weeks ormore (Oechel et al., 1997). Earlier snow accumulation anda deeper snowpack can effectively insulate soils from coldair temperatures (Zhang, 2005; Yi et al., 2015). Soil moisture can further delay soil freezing due to large latent heatrelease with soil-water phase change, where soil temperatures can persist near 0 ◦C (i.e., the zero-curtain period) forup to several weeks or more during the late fall and earlywinter seasons. The zero curtain can sustain soil microbialactivity and has been shown to be closely correlated withsoil respiration during the early cold season (Zona et al.,2016; Euskirchen et al., 2017). Highly organic soils and peat(e.g., SOC > 25 kgCm−2), prevalent in the ABZ, can act asstrong insulators during the summer thaw season and canalso have a significant impact on the soil thermal regime andhydrologic processes due to their distinct hydraulic and thermal properties (Lawrence and Slater, 2008; Rawlins et al.,2013)